The role of epigenetic DNA modifications, like changes in the DNA methylation and histone modification, in cellular senescence and aging are still poorly understood. Previous studies indicated that global DNA methylation decreases during aging in many tissue types while ageing associated focal hypermethylation of CpG-islands may lead to transcriptional silencing of certain genes. The fidelity with which DNA methylation patterns in mammals are inherited after each cell division is ensured by the DNA-methyltransferases (DNMTs) that are classified as maintenance methyltransferase (DNMT1) and de novo methyltransferase (DNMT3a, DNMT3b) respectively. Furthermore methylated DNA-binding proteins (MBD) are involved in chromatin remodelling and epigenetic regulation of gene expression.

We have investigated potential changes in the gene expression patterns of DNMT1, DNMT2, DNMT3a and b, MBD1, MBD2, MBD3, MBD4 and MeCP2 in relationship to UVB-induced cellular senescence in vitro as well as intrinsic and extrinsic (UVB- induced) skin ageing in vivo. As model systems we used human normal dermal fibroblasts (NHDF) for in vitro experiments and skin tissue biopsies derived from C57/BL6 mice the in vivo studies. NHDF and mouse skin biopsies were harvested at defined intervals (4, 8, 12, 24, and 48h) after a single dose of UVB exposure and then subject of nucleic acid and protein extraction, followed by molecular analysis. We also performed long-term experiments after chronic repetitive UVB exposure with doses of UVB ranging from 1 to 10 mJ/cm2. Transcript levels of each of the DNMT's and MBD's as well as known UVB target genes COX2, CDKN2a (p16^INK4A), CDKN1a (p21^WAF) were assessed by real-time PCR.

Short time exposure of mice to a single dose of 240 mJ/cm² UVB showed time- dependent increase in the mRNA levels of the 3 genes in skin biopsies. Elevated transcript levels were also detected for DNMT3b while DNMT2 mRNA expression decreased after UVB exposure. In cultured NHDF single dose of 10 mJ/cm² UVB induced increased transcript level of COX2 and CDKN1a but not CDKN2a, while transcript levels of DNMT2 and DNMT3a decreased after irradiation. Chronic repetitive exposure of mice to UVB over 4 months resulted in elevated levels of COX2, CDKN1a and CDKN2a in the skin, while the investigated DNMTs and MBDs did not show obvious changes in their mRNA expression levels. Regarding the intrinsic skin ageing (no UVB irradiation), we found higher expression of COX2 and CDKN2a in skin biopsies of old versus young mice while the transcript levels of DNMT1, DNMT2, MBD3, MeCP2 appeared to be increased during intrinsic ageing.

Taken together, our results suggest that expression levels of certain DNMTs and MBDs might be involved in regulation of intrinsic skin ageing as well as in the cellular response induced by UVB exposure in vivo and in vitro. Additional studies are under way to further elucidate the role of epigenetic alterations in skin ageing.